Considerable attention has been recently focused on a hybrid vehicle and an electric vehicle as an environmentally friendly vehicle.
This hybrid vehicle uses, in addition to the conventional engine, a DC power supply, an inverter, and a motor driven by the inverter as a power source. That is, while the hybrid vehicle is powered by driving the engine, it is also powered by converting the DC voltage from the DC power supply into an alternating current (AC) voltage by the inverter and by using the converted AC voltage to rotate the motor. The electric vehicle also uses a DC power supply, an inverter, and a motor driven by the inverter as a power source.
Japanese Patent Laying-Open No. 2001-295676 discloses that, in a hybrid vehicle, an angular acceleration of the drive shaft is used to detect the slipping state and to limit the torque.
Immediately after a running vehicle runs on to a protrusion on the road surface, for example, a road shoulder, a fallen object, and the like, a slipping state is caused in which the tire spins. Since the road surface resistance is eliminated in the slipping state, the rotation speed of the wheel is increased if the wheel is rotated without changing the torque.
Since the output power is proportional to the torque×the rotation speed, the slipping state causes a lot of electric power to be consumed in the motor which drives the wheels. Accordingly, the control is performed such that more electric power may be supplied to the motor.
On the other hand, after the slipping state is over, the tire comes into contact with the road surface again to bring about a gripping state, in which the rotation speed of the tire is rapidly decreased due to friction with the road surface. The rotation speed of the motor also rapidly decreases with decrease in the rotation speed of the tire.
It is also contemplated that, in the hybrid vehicle and the like, the DC voltage from the power supply is boosted by a boost converter, and the boosted DC voltage is converted into an AC voltage by the inverter to drive the motor.
In such a configuration including the boost converter, when the rotation speed of the motor rapidly decreases, excessive electric power may be supplied from the boost converter to the inverter. Therefore, there is a need to lower the target voltage of the boost converter.
Japanese Patent Laying-Open No. 2001-295676 does not disclose the control of the boost converter performed in the case of such a change from the slipping state to the gripping state.